Diel changes of dissolved oxygen (DO) and pH were measured during 1981–1982 in Pool 1, a shallow reservoir in southeast Missouri. Diel changes in spring and fall were about half those of summer when extensive macrophyte biomass apparently had a strong influence on dynamics of DO and pH. During summer, daily pulses averaged 5.45 mg/L DO and 0.88 pH units, and extreme diel changes were 10–14 mg/L DO and 2–3 pH units. Ambient values of DO and pH were directly related to each other as was the magnitude of their daily changes because in the low alkalinity water of Pool 1, pH as well as DO responded to photosynthesis and respiration. Magnitudes of DO and pH changes at night were strongly related to fluctuations during the day, suggesting that community respiration was in a steady state relationship with photosynthesis over time.